A distinguishing feature of the mammalian neocortex is its remarkable ability to change over a lifetime, especially during early development. Thus, the functional organization and connectivity of each individual's brain is tailored to the physical parameters of a specific environment, permitting behavior to be uniquely optimized for a given sensory milieu. Such plasticity plays an integral role in shaping the brains of normal humans as well those who suffer from severe visual impairments due to retinal abnormalities or cortical lesions that occur at various stages of development. This proposal will investigate the extent of cortical plasticity following experimentally induced manipulations to the visual system during development. Our first objective is to examine the alterations in sensory mediated behavior, as well as changes in the functional organization, connectivity and cellular composition of the neocortex that result from one of two induced neural insults: 1) loss of neocortex that would normally develop into visual cortex; 2) loss of visual input normally provided by the retina. The second objective is to determine if early, pervasive sensory enhancement can be used to direct the functional reorganization of the neocortex and optimize sensory mediated behavior. Manipulations will be made at one of three developmental milestones: 1) Before retinal ganglion cell axons enter the diencephalon and before thalamocortical afferents have reached the cortex. 2) Before eye opening, after thalamocortical afferents have innervated the neocortex, but before axonal pruning and the completion of cortical development. 3) Just after the eyes have opened, when retinofugal and thalamocortical development is established and the subventricular zone and all six cortical layers are present. These animals will be exposed to either a normal or to a tactilely (for bilateral enucleates) or visually (for cortical lesions) enhanced environment. Our animal model, the short-tailed opossum (Monodelphis domestica) is born prematurely, allowing ex-utero manipulations to the nervous system at developmental time points that would be in-utero in other mammals. After the animals have reached maturity we will use behavioral testing combined with electrophysiological and neuroanatomical techniques to examine sensory discrimination, the functional organization and neural response properties of re-organized cortex, cortical and thalamic connectivity, and the cellular composition including neuronal number and density of re-organized cortex. These studies, which are novel in their scope, provide an opportunity to translate detailed knowledge gained at the cellular and systems level to produce significant therapeutic interventions designed to direct multisensory plasticity, and optimize sensory mediated behavior following loss of vision.